I was at the Tuner Evolution last weekend and I saw something that puzzled me, I hope someone on here will be able to shed some light on this - I saw a SilverStone S with a big turbo. The weird thing was there were 2 blow off valves before the turbo in the exhaust manifold. I've heard argument of BOV placement before or after the intercooler but never before the turbo. What would be the purpose of them? Why 2?

 

Those were wastegates not BOV's. Same basic function, only it releases exhaust gasses. This way you can stop the turbo from spooling and "hover" at a certain PSI.

This guy you mentioned had two because of the style of manifold. It is called a "twin-scroll" manifold.

 

Don't waste gates usually recirculate? Being that they are set up before the turbo wouldn't they be less accurate since it's taking the pressure before the turbo? I guess the pressure before the turbo will give you a good idea of what the turbo can boost to but somehow it feels like it would be less accurate for some reason.

 

That is the point. If you decsease pressure before the turbo, you can stop it from building more boost. Sometimes the dumptubes from the wastegates are recirculated to the down pipe but they don't have to. Many chose to vent to the atmoshphere through the hood, fender, or down under the car.

A BOV is used to release ALL positive air pressure in the intake piping when you let off the throttle and the intake manifold closes.